“In humans, fear is something that persists on a longer timescale than a simple escape reflex,” says William Gibson of the California Institute of Technology in Pasadena, California. “Our observations suggest flies have a persistent state of defensive arousal, which is not necessarily fear, but which has some similarities to it.”

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Evasive action

Gibson and his colleagues exposed fruit flies to overhead shadows resembling aerial predators, such as birds. The more shadows they were exposed to, the more the flies resorted to evasive behaviour, such as hopping, jumping or freezing.

When the shadow passed over once per second, by the time the shadow had fallen 10 times, the average running speed of the flies had doubled, for example. Their average number of hops trebled after just two passes.

They also offered starved flies food, and part way through the meal threatened them with shadows. The more often the meal was interrupted, the longer the flies took to return to their meal after flying away.

“The more shadows we used to disperse them from the food, the longer they took to ‘calm down’ and return to the food,” says co-author David Anderson, also at Caltech.

The researchers compare this to reactions of humans witnessing a shooting. The fear can outlast the gunshot itself, increasing if more shots are fired, and would arise whatever else the person was doing at the time of the shooting.

Comparable behaviour

“The results provide compelling evidence for anxiety-type responses in flies that resemble comparable behaviours in mammals,” says Jaak Panksepp at Washington State University in Pullman.

Gibson and his colleagues now plan to pick apart the genetics and neurophysiology behind these responses, and perhaps even use flies as models to study human fear. “The fruit fly may be a good model for emotion states because of the relative simplicity of its nervous system,” says Gibson.

Whether they would be a good model is still very much up for debate, because not everyone agrees we can attribute emotions to animals. “The fly fear system may not qualify as an emotion,” says Mathias Clasen, from Aarhus University in Denmark. “But flies do seem to be motivated by an aversive state that occurs in response to a potential threat in the environment, just like so many other organisms, including us.”

“It shouldn’t surprise us to find something resembling fear in flies, as it’s a mechanism that evolved over millions of years to protect organisms from harm, so its retention across those millions of years attests to its efficiency,” he says. “If it didn’t work, natural selection would have got rid of it.”